Photoelectrophoretic imaging system

ABSTRACT

A photoelectrophoretic system is disclosed which utilizes a pneumatic operated circuit to apply an imaging suspension onto injecting and blocking electrodes, to control the agitation of the ink between the electrodes and to remove excess ink from the electrodes. The blocking electrodes are either a skid-type member or a roller-type member with both being supported to make contact with an injecting electrode.

I United States Patent 3,619,053

[72] Inventor Kallis H. Mannik [56] References Cited Rochester UNITED STATES PATENTS [21] P 3,383,993 5/1968 Shu-Hsiung Yeh 355/10 x 1969 3 484 162 12/1969 Clark 355/3 [45] Patented Nov. 9, i971 f (73] Assignee Xerox Corporation Primary Examiner-Samuel S. Matthews Rochester, N.Y. A.i.$i.\ltl!llE.\(1!I1int'fM0nl'0 H. Hayes Attorneys-James J. Ralabate. David C. Petre and Michael H.

Shanahan ABSTRACT: A hotoelectro horctic s stem is disclosed [54] fgg P Q Q IMAGING SYSTEM which utilizes a pn eumatic opefated circuit to apply an imagalms, 3 Drawing Figs.

ing suspension onto in ecting and blocking electrodes, to con- [52] U.S.Cl 355/3, trol the agitation of the ink between the electrodes and to 355/10, 355/17 remove excess ink from the electrodes. The blocking elec- [51] Int. Cl G03g 15/00 trodes are either a skidtype member or a roller-type member [50] Field of Search 355/3. 10. with both being supported to make contact with an injecting l7 electrode.

PHOTOELECTROPI-IORETIC IMAGING SYSTEM BACKGROUND OF THE INVENTION This invention relates to imaging systems and in particular to a novel photoelectrophoretic imaging system.

A detailed description of the photoelectrophoretic imaging process is given in U.S. Pat. Nos. 3,384,488, 3,384,566, and 3,383,993. The photoelectrophoretic process forms images from an ink or imaging suspension comprised of an insulating liquid carrier having photosensitive particles suspended within it. An image is formed by placing the ink between injecting and blocking electrodes, one of which is at least partially transparent, by exposing the ink to a light image through the transparent electrode and by establishing an electric field between the electrodes. One theory of operation is that the particles have a net charge when suspended in the liquid carrier and hence are attracted to the electrodes under the influence of the electric field. Those particles attracted to the injecting electrode and exposed to the light image experience an apparent change in charge polarity and are attracted to the blocking electrode thereby creating a positive image on the injecting electrode and a negative image on the blocking electrode. The blocking electrode is so named because it has the characteristic of preventing the charge exchange with the particles attracted to it. This characteristic is provided by coating the blocking electrode with a dielectric material. The process is either monochromatic or polychromatic depending upon whether the particles suspended within the liquid carrier are sensitive to the same or different portions of the light spectrum. A full color system is obtained, for example, by using cyan, magenta and yellow particles which are sensitive to red, green, and blue light respectively.

It has been found for some photoelectrophoretic inks that the quality of the images produced by the photoelectrophoretic process are improved if the ink experiences dynamic forces during the exposure to the light image and during the application of the electric field. Consequently, it is common to form images by moving, for example, the blocking electrode over the injecting electrode to impart at least some amount of agitation to the ink. The most common embodiment for this operation is to shape at least one of the electrodes in a roller configuration so that the ink is agitated at least to some degree by the rotational and translational movement of the roller.

Accordingly, it is an object of this invention to improve the photoelectrophoretic process by providing novel means for agitating the ink between the injecting and blocking electrodes. Still another object of this invention is to devise a novel configuration for a photoelectrophoretic system electrode. It is also an object of the invention to improve the means for obtaining a layer of ink between the injecting and blocking electrodes and for controlling the amount of agitation of the ink placed between the electrodes.

A further object of the invention is to devise novel means for applying ink to the injecting and blocking electrodes of a photoelectrophoretic system and for removing the unused or excess ink from the electrodes.

These and other objects of this invention are realized by employing a pneumatic system for applying ink under pressure into a nip formed between moving a stationary electrodes, the injecting and blocking electrode of the system. The pneumatic apparatus is also used to remove excess or unused ink from the electrodes. The moving electrode is forced against the sta tionary electrode to form the nip. The ink applied to the nip is pushed by the moving electrode across the stationary electrode. The photosensitive particles in this ink are pulled into the nip by the electric field established between the electrodes and the pressure exerted on the ink by the pneumatic system. Images are formed from the photosensitive particles at the nip that are exposed to electromagnetic radiation. The supply of ink available at the nip and the degree or amount of ink agitation is controlled by varying the pressure exerted of the ink. Directional flow control valves are used to switch between various pressure levels to start and stop the flow of ink and to maintain a desired quantity of ink available for depositing onto the electrode. In one embodiment of the invention the moving electrode is a sliding or skid member that makes contact with a stationary electrode. The ink is applied in front of the nip formed between the two electrodes and the excess ink is removed through a plurality of pores in the skid electrode that are connected to a source of negative pressure i.e., a pressure below atmospheric pressure. Another embodiment uses a vibrator to oscillate the skid and thereby impart additional agitation to the ink. The contact skid electrode constitutes a novel configuration for a photoelectrophoretic imaging system electrode. In a third embodiment, the pneumatic inking mechanism is combined with a moving electrode that is a roller member.

DESCRIPTION OF THE DRAWINGS Other objects and features of the invention will be apparent from a further reading of the specification and from the drawings which are:

FIG. 1 is a perspective view of a schematic representation of photoelectrophoretic imaging system according to the present invention utilizing a novel skid electrode;

FIG. 2 is a schematic representation of a cross section view of a portion of a photoelectrophoretic imaging system according to the present invention utilizing a roller electrode;

FIG. 3 is a schematic representation of a cross section view of a portion of a photoelectrophoretic imaging system according to the present invention illustrating yet another skid electrode and the fluid operated (i.e., pneumatic) circuit for applying and removing ink from the electrodes.

DESCRIPTION OF THE INVENTION The photoelectrophoretic system illustrated in FIG. 1 includes the exposure mechanism 1, the stationary electrode 2, the moving electrode 3, and the inking means 4. In this and the other embodiments of the invention the stationary electrode 2 is the injecting electrode for the photoelectrophoretic process and includes an optically transparent glass 6 coated with an optically transparent conductive material 7 such as tin oxide. Tin oxide coated glass of the type described is commercially available under the name NESA glass. The moving electrode 3 is the blocking electrode for the system and includes the dielectric member 8 composed of a material having a volume resistivity preferably in the order of l0 (or greater) ohms centimeters and of the conductive member 9 which is preferably a resilient material such as a conductive rubber used to give flexibility to the skid. The moving electrode 3 is supported such that the dielectric material 8 contacts the conductive layer 7 forming a nip 19. The electrodes have terminals 11 and 12 for coupling to the battery or voltage 13 to establish the electric field between them.

The exposure mechanism 1 includes the light source 14, the transparency 16, and the lens 17. The function of the exposure mechanism is to expose the ink to electromagnetic radiation. The exposure mechanism projects a light image, i.e., electromagnetic radiation in imagewise configuration, of the transparency through the transparent stationary electrode 2 to ink in the nip between the two electrodes. A positive image of the entire transparency is formed on the stationary electrode in the areas of the electrode onto which the light image is projected by closing switch 18 to apply the voltage of battery 13 to the electrodes thereby establishing the required electric field and by passing the moving electrode from left to right across the stationary electrode.

The terms front" and behind" as used herein in connection with the moving electrode refer to the direction of movement of the moving electrode. In the case of left to right travel of the moving electrode the term front" refers to a point at or to the right of the right hand or leading edge 20 of nip l9 and the term behind means any point to the left of edge 20.

The inking means 4 includes the manifold 21 and the fluid control circuit 22. The manifold has two parallel longitudinal conduits 23 and 24 within its body extending substantially the entire width of the moving electrode. Inlet conduits 23 along with conduit 27 comprise a feed conduit through which ink flows onto the electrodes. Conduit 27 is formed between the leading edge of the moving electrode and the squeegee 26 which extends substantially the entire width of the moving electrode. The feed conduit terminates in front of the moving electrode, in this embodiment the skid member 28, and directs the ink spray into the nip 19.

The manifold outlet conduit 24, along with the plurality of pores 30 formed in the skid behind the leading edge 20, comprise a vacuum conduit through which the excess ink is removed from the skid in the stationary electrode. The pores extend along the entire width of the skid and are relatively small and closely spaced together. When a vacuum is drawn on the pores the excess ink on the electrodes is transported away. 7

The ink referred to as excess ink is that portion of the ink not used to form the image on the stationary electrode. Earlier the ink was described as an insulating liquid carrier having photosensitive particles suspended within it. It is those particles remaining on the stationary electrode after exposure to the light image which make up the desired positive image. The liquid carrier between the electrodes and the particles adhering to the moving electrode therefore represents the excess ink. The force exerted on the ink by the vacuum is selected to be sufficient to remove at least the liquid carrier without overcoming the bond of the particles adhering to the stationary electrode. It is believed that this allows the particles adhering to the skid electrode to be removed because of the additional forces acting on these particles by the washing action of the liquid carrier being drawn into the pores.

Turning now to FIG. 3, the fluid control circuit 22 includes the inlet and outlet chambers or tanks 31 and 32, the inking tank 33, the solenoid operated valves 34 and 36, the float 37, the switch 38 and the associated fluid conduits or rubber tubing 39. The inlet and outlet chambers are rigidly connected to the manifold 21 forming a part thereof and are in fluid communication with the inlet and outlet conduits 23 and 24 of the manifold respectively. The chambers 31 and 32 are fully enclosed containers with the inlet chamber serving as a means for holding the supply of ink in readiness for the feeding operation and the outlet chamber serving as a means for collecting the excess ink removed from the electrodes.

The ink collected in the outlet chamber may be recirculated to the inlet chamber if so desired by coupling the two chambers together by conduit 41. The one-way valve 42 is used to limit the direction ofink flow solely to the inlet chamber. The one-way valve 43 is positioned adjacent the orifice in the outlet chamber which is in fluid communication with the manifold outlet conduit 24 to limit the direction of ink flow solely to the outlet chamber. The inlet chamber 31 receives its supply of ink from the ink tank 33 through orifice 44 via conduit 45. The one-way valve 46 is positioned at orifice 44 to limit the direction of the ink flow solely to the inlet chamber.

Before proceeding further, a few definitions of terms will be helpful. A negative pressure gas source refers to a cylinder or other means which is partially evacuated of gases to lower its internal pressure below atmospheric pressure. Similarly, a positive pressure gas source refers to a cylinder or other means containing a compressed gas to create an internal pressure greater than atmospheric pressure. The term vacuum refers to a negative pressure, i.e., a pressure below atmospheric, and not to an absolute void. The term fluid is intended to encompass both gases and liquids. The gases referred to are preferably those commonly found in the atmosphere and identified generally as air.

The outlet chamber 32 is coupled to a negative pressure gas source -b by means of the orifice 47 and conduit 48. The magnitude of the pressure b is selected to be sufficient to draw the excess ink from the electrodes into the outlet chamber.

The inlet chamber 31 is coupled to either the negative pressure gas source b or the positive pressure gas source +a by virtue of the solenoid operated, spring biased two-way two positioned valve 34. The intake orifice 49 of valve 34 is coupled to the positive pressure source +a by conduit 51 and intake orifice 52 is coupled to the negative pressure source -b by conduits 53 and 48. The exit orifice 54 of valve 34 is coupled to the inlet chamber orifice 56 by conduit 57. Valve 34 is held at its left valve position when solenoid 58 is energized thereby coupling the intake orifice 49 to the exit orifice 54 to apply the positive pressure gas source +a to the inlet chamber. The positive pressure +a forces the ink into the feed conduit (the manifold inlet conduit 23 and conduit 27) to feed the ink onto the electrodes. Valve 34 is switched to its right valve position by spring 59 when solenoid 58 is deenergized thereby coupling the intake orifice 52 to exit orifice 54 to apply the negative pressure gas source -b to the inlet chamber. The negative pressure b ink in the inlet chamber thereby preventing the ink from being sprayed onto the electrodes. Therefore it is apparent that the depositing of ink onto the electrodes is controlled by switching valve 34 between its right and left valve positions. This switching operation is effected by the manual operated switch 61 which couples an activating electrical energy source (battery 62) to the solenoid 58 when it is closed.

The pressure regulator 60 coupled to conduit 51 is a device for varying the magnitude of the positive pressure +0. The regulator 60 provides means for varying the force exerted on ink at the electrodes.

The ink tank 33 is a fully enclosed container used to store a supply of ink for the system. As mentioned earlier, tank 33 is coupled to the inlet chamber 31 via conduit 45 and one way valve 46. The tank is also coupled by conduit 63 to the exit orifice 64 of the solenoid operated, spring biased, two-way, two positioned spool valve 36. The intake orifice 66 of valve 36 is coupled by conduit 67 to the positive pressure gas source +0 and the intake orifice 68 is coupled by conduits 69 and 48 to the negative pressure gas source b. The valve 36 is held at the left valve position when solenoid 71 is energized thereby coupling intake orifice 66 to the exit orifice 64 to apply the positive pressure +0 to the ink tank. The positive pressure +c is greater than either of the pressures +11 or b and therefore forces the ink from tank 33 past the one-way valve 46 into the inlet chamber 31. The valve 36 is normally held at its right valve position by the spring 72 and is switched to that position when the solenoid 71 is deenergized. When valve 36 is at the right valve position, intake orifice 68 is coupled to exit orifice 64 to apply the negative pressure b to the ink tank. The negative pressure -b is less than the positive pressure +a, therefore the ink cannot flow pass the one-way valve 46 when the positive pressure +a is applied to the inlet chamber and the negative pressure b is applied to the ink tank. If the inlet chamber has the negative pressure +b applied to it while the b pressure is applied to the tank 33, ink still will not flow pass the one-way valve 46 because valve 46 requires a definite pressure differential across it before it will open. Accordingly, it is apparent that the flow of ink from the tank 33 to the inlet chamber 31 is controlled by switching valve 36 between its left and right valve positions.

The float 37 and switch 38 are used to control the switching operation of valve 36 and therefore to control the level of ink in the inlet chamber 31. The float 37 is coupled to rod 73 which is in turn pivotally connected to the chamber by stand 74. Switch 34 is mounted on rod 73 and is a liquid mercury contact switch which couples an electrical source (battery 76) to solenoid 71 whenever it is tilted from a predetermined orientation such as horizontal. Therefore, when the ink level in chamber 31 falls below a predetermined level (normally when ink flows to the electrodes) the float 37 falls thereby rotating rod 73 downward. The downward rotation of rod 73 causes the switch 38 to energize solenoid 72 by completing a circuit between the solenoid and the battery 76. With solenoid 72 energized the pressure +c is applied to the ink tank causing ink to enter the inlet chamber through the one-way valve 46. when the ink level is returned to the predetermined level, the float moves upward causing the switch 39 to break the circuit between battery 76 and solenoid 72 thereby switching valve 36 back to its spring biased or right valve position. At the right valve position b pressure is applied to the ink tank 33 thereby preventing the flow of ink to the inlet chamber.

The structure of the manifold 26 and moving electrodes are essentially identical for the embodiments in FIGS. 1, 2, and 3. Consequently, parts common to each embodiment are identified by like reference numbers. In the embodiments of FIGS. 1 and 2, the skids 28 and 77 are rigidly connected to the manifold 21 by appropriate means. In all three embodiments the squeegee 26 is rigidly connected by appropriate means to the manifold as is the support member 78 used to prevent the squeegee from buckling when the manifold is moved across the stationary electrode. The skid 77 in FIG. 3 has a mechanical or acoustic vibrator 79 inserted within the conductive member 9 to oscillate the skid and thereby impart agitation to ink between electrodes. The vibrator 79 is powered by appropriate means not shown in the drawings to simplify and therefore clarify the present description.

The moving electrode for the embodiment shown in FIG. 2 is the roller 81 which is journaled for rotation of shaft 82. Shaft 82 is in turn coupled to the manifold 21 by appropriate structure not shown in order to simplify and therefore clarify the present description. The roller includes the inner core 83 of conductive material and an outer layer 84 mounted thereto of a dielectric material having a resistivity in the order of (or greater) ohms per cubic centimeter. The squeegee 86 is rigidly connected to the manifold 21 and contacts the periphery of the roller near the top of the roller so as to form along with squeegee 26 a portion of the feed conduit used to feed the ink under pressure onto the electrodes. A porous member 87 is connected to the manifold over the intake 88 to the outlet chamber 24 and extends down to the periphery of the roller behind the squeegee 86. The porous member along with the outlet conduit 24 of the manifold forms the vacuum conduit for removing excess ink from the roller.

Several modifications can be made to the above described embodiments without departing from the spirit or scope of the present invention. By way of example, the functional role of the moving and stationary electrodes can be reversed so that the moving electrode is the injecting electrode and the stationary electrode is the blocking electrode. The electrode described as stationary can be moved in addition to the moving electrode and both electrodes can be shaped in a drum or roller type configuration. The application of field and light need not be simultaneous hence neither electrode need be transparent. The light image can be projected through the moving rather than the stationary electrode and the polarities of the various voltages can be reversed.

What is claimed is:

l. Photoelectrophoretic imaging apparatus comprising first and second electrodes supported to contact one another at a nip and supported for relative movement having means for coupling to a voltage source to establish an electric field between them at the nip,

exposure means for exposing photoelectrophoretic ink on at least one of said electrodes to activating electromagnetic radiation, and

inking means for forcing photoelectrophoretic ink into said nip and for removing excess ink from the electrodes including a feed conduit adjacent said nip coupled to positive pressure means for exerting positive pressure on ink in the conduit to force ink into the nip and a vacuum conduit adjacent at least one electrode coupled to negative pressure means for exerting a negative pressure on ink to remove ink from an electrode whereby images are formed from ink at the nip exposed to electromagnetic radiation and subjected to electric field.

2. The apparatus of claim 1 wherein said first electrode includes a skid member supported for sliding movement relative to said second electrode.

3. The apparatus of claim 1 wherein said first electrode includes a roller member supported for rolling movement relative to said second electrode.

4. The apparatus of claim 1, wherein said inking means further includes storage means for storing photoelectrophoretic ink and means for coupling to said feed conduit and positive pressure means for feeding ink under pressure from said storage means to said nip.

5. The apparatus of claim 1 wherein said inking means includes a manifold coupled to one of said electrodes for feeding ink to said feed conduit and said nip and a vacuum conduit coupled to a negative pressure source for removing excess ink from said electrodes.

6. The apparatus of claim 1 wherein at least a portion of said feed conduit is formed between an edge of said first electrode and a squeegee member coupled to said first electrode and contacting the second electrode.

7. The apparatus of claim 1, wherein said first electrode is a skid member having vibration means of oscillating said skid to agitate ink between the skid and second electrode.

8. Photoelectrophoretic imaging apparatus comprising a skid member electrode and a second electrode supported to contact one another at a nip and supported for movement to contact one another having means for coupling to a voltage source to establish an electric field between them at the nip,

exposure means for exposing photoelectrophoretic ink on at least one of said electrodes to activating electromagnetic radiation,

a manifold having inlet and outlet chambers for containing ink,

a feed conduit coupled to said inlet chamber and terminated adjacent said skid member,

a vacuum conduit coupled to said outlet chamber and terminated adjacent said skid member,

means for coupling said inlet chamber to a positive pressure fluid source for forcing ink through said feed conduit into the nip, and

means for coupling said outlet chamber to a negative pressure fluid source for removing excess ink from the electrodes through fluid source for removing excess ink from the electrodes through said vacuum conduit.

9. Photoelectrophoretic imaging apparatus comprising a roller member electrode and a second electrode supported to contact one another at a nip and supported for movement relative to one another having means for coupling to a voltage source to establish an electric field between them at the nip,

exposure means for exposing photoelectrophoretic ink on at least one of said electrodes to activating electromagnetic radiation,

a manifold having inlet and outlet chambers for containing ink,

a feed conduit coupled to said inlet chamber and terminated adjacent said roller member,

a vacuum conduit coupled to said outlet chamber and terminated adjacent said roller member,

means for coupling said inlet chamber to a positive pressure fluid source for forcing ink through said feed conduit into the nip between electrodes, and

means for coupling said outlet chamber to a negative pressure fluid source for removing excess ink from said electrodes through said vacuum conduit.

10. The apparatus of claim 9 wherein said vacuum conduit includes a porous member coupled to said manifold having a plurality of pores terminating at the periphery of said roller member for removing excess ink therethrough.

11. The apparatus of claim 9, wherein said inking means further includes a first squeegee coupled to said manifold and positioned adjacent said roller member for contacting said second electrode and a second squeegee member coupled to said manifold contacting the periphery of said roller member with said first and second squeegee members forming a portion of said feed conduit.

12. A photoelectrophoretic imaging apparatus comprising first and second electro one of which is at least partially transparent and at least one of which is supported for movement relative to the other with both having means for coupling to a voltage source to establish an electric field between them,

exposure means for exposing ink located between the electrodes to a light image to form an image from. the ink exposed to the light image and subjected to the electric field,

a manifold having inlet and outlet chambers for containing ink and having said first electrode coupled thereto,

feed conduits coupled to said inlet chamber and terminated 

2. The apparatus of claim 1 wherein said first electrode includes a skid member supported for sliding movement relative to said second electrode.
 3. The apparatus of claim 1 wherein said first electrode includes a roller member supported for rolling movement relative to said second electrode.
 4. The apparatus of claim 1, wherein said inking means further includes storage means for storing photoelectrophoretic ink and means for coupling to said feed conduit and positive pressure means for feeding ink under pressure from said storage means to said nip.
 5. The apparatus of claim 1 wherein said inking means includes a manifold coupled to one of said electrodes for feeding ink to said feed conduit and said nip and a vacuum conduit coupled to a negative pressure source for removing excess ink from said electrodes.
 6. The apparatus of claim 1 wherein at least a portion of said feed conduit is formed between an edge of said first electrode and a squeegee member coupled to said first electrode and contacting the second electrode.
 7. The apparatus of claim 1, wherein said first electrode is a skid member having vibration means for oscillating said skid to agitate ink between the skid and second electrode.
 8. Photoelectrophoretic imaging apparatus comprising a skid member electrode and a second electrode supported to contact one another at a nip and supported for movement relative to one another having means for coupling to a voltage source to establish an electric field between them at the nip, exposure means for exposing photoelectrophoretic ink on at least one of said electrodes to activating electromagnetic radiation, a manifold having inlet and outlet chambers for containing ink, a feed conduit coupled to said inlet chamber and terminated adjacent said skid member, a vacuum conduit coupled to Said outlet chamber and terminated adjacent said skid member, means for coupling said inlet chamber to a positive pressure fluid source for forcing ink through said feed conduit into the nip, and means for coupling said outlet chamber to a negative pressure fluid source for removing excess ink from the electrodes through fluid source for removing excess ink from the electrodes through said vacuum conduit.
 9. Photoelectrophoretic imaging apparatus comprising a roller member electrode and a second electrode supported to contact one another at a nip and supported for movement relative to one another having means for coupling to a voltage source to establish an electric field between them at the nip, exposure means for exposing photoelectrophoretic ink on at least one of said electrodes to activating electromagnetic radiation, a manifold having inlet and outlet chambers for containing ink, a feed conduit coupled to said inlet chamber and terminated adjacent said roller member, a vacuum conduit coupled to said outlet chamber and terminated adjacent said roller member, means for coupling said inlet chamber to a positive pressure fluid source for forcing ink through said feed conduit into the nip between electrodes, and means for coupling said outlet chamber to a negative pressure fluid source for removing excess ink from said electrodes through said vacuum conduit.
 10. The apparatus of claim 9 wherein said vacuum conduit includes a porous member coupled to said manifold having a plurality of pores terminating at the periphery of said roller member for removing excess ink therethrough.
 11. The apparatus of claim 9, wherein said inking means further includes a first squeegee coupled to said manifold and positioned adjacent said roller member for contacting said second electrode and a second squeegee member coupled to said manifold contacting the periphery of said roller member with said first and second squeegee members forming a portion of said feed conduit.
 12. A photoelectrophoretic imaging apparatus comprising first and second electrodes one of which is at least partially transparent and at least one of which is supported for movement relative to the other with both having means for coupling to a voltage source to establish an electric field between them, exposure means for exposing ink located between the electrodes to a light image to form an image from the ink exposed to the light image and subjected to the electric field, a manifold having inlet and outlet chambers for containing ink and having said first electrode coupled thereto, feed conduits coupled to said inlet chamber and terminated adjacent said first electrode, vacuum conduits coupled to said outlet chamber and terminating adjacent said first electrode, flow control means for coupling said inlet chamber to a positive pressure fluid source for forcing ink through said feed conduit onto said electrodes and for coupling said inlet chamber to a negative fluid source to inhibit the flow of the ink, and vacuum means for coupling said outlet chamber to a negative pressure fluid source for removing excess ink from said electrodes. 